Method for the preparation of a laminate and a laminate obtainable thereby

ABSTRACT

A laminate of two films is prepared by partially or completely covering a surface of a first film with a layer of ink comprising a catalyst suitable for accelerating curing of an adhesive for films and, subsequently, applying such an adhesive between the layer of ink and a second film before curing the adhesive.

The present invention relates to a method for the preparation of aquick-curing laminate and a laminate obtainable thereby.

Plastic film is a frequently used material for the packaging of numerousarticles and products. Within many fields of application, therequirements for such plastic films are of a character that cannot bemet by one film alone. Consequently, various laminates of films havebeen developed with properties that in many respects meet therequirements better.

Usually, adhesives are used to attach separate films to each other, thusgenerating a laminate. Various requirements are made for such adhesives,e.g. adherence, heat resistance, transparency, and it has been provedthat, inter alia, polyurethane-containing adhesives may be used for theadhesion of the separate films of a laminate, thus obtaining therequested properties of the laminate.

As far as polyurethane adhesives are concerned, it is problematic,however, that the curing time is long and that a laminate has to age,possibly at a comparatively high temperature and in a properlyventilated room, to ensure the adhesive curing.

In order to reduce the curing time of the adhesives employed, variousmethods have been used, including the addition of a catalyst for thecuring reaction.

When using a catalyst, the simplest way to accelerate the curing of apolyurethane adhesive is to add the catalyst directly into thepolyurethane adhesive, immediately before the adhesive is applied to afilm that is to be attached to another film. On an industrial scale,this is not efficient, however, since to some extent the adhesivecomposition will have cured before use. The curing will cause anincreased viscosity and a subsequently more difficult application to thefilm, whereby the application quantity cannot be controlled. A limitedspace of time is available for the administration so the adhesivecomposition, the so-called pot-life, and, in consequence, productiondisturbances may cause glue vessels and application devices having to becleared of cured adhesives.

To overcome said problem of the low pot-life, in EP-A1-0152102 it issuggested to use a urethane adhesive containing micro capsules with acatalyst for the curing. Such micro capsules will be destroyed when thelaminate film is attached by means of pressure rollers, whereby theencapsulated catalyst will be released.

The disadvantage of this method is that a homogenous distribution of thecatalyst in the glue layer cannot be obtained, since the catalystconcentration will peak in the immediate vicinity of the destroyedcapsule. Furthermore, such capsules will remain in the adhesive as asuspension and therefore precipitate when stored.

Another suggestion to bring a polyurethane adhesive into contact withthe catalyst is disclosed in EP-A1-0586843. In this, a method for thepreparation of a laminate film is disclosed, wherein at least two filmsare attached to each other by means of a polyurethane adhesive, theadhesive layer or the adhesive-free film being humidified with anaqueous solution bearing at least one catalyst, which accelerates thecuring reaction of polyurethane adhesives. The catalyst solutionhumidification is problematic, however, since it will be difficult toobtain a homogenous distribution of the solution on a film, e.g. apolyethylene or polypropylene film, even when using a surfactant in thesolution, inter alia, because suitable equipment for a homogenousapplication of the catalyst solution all over the film is not available,so that in certain sections of the prepared laminate water willaccumulate, which leads to an isocyanate conversion to the correspondingamine compound, thus involving the risk of subsequent amine compoundmigration. Therefore, whenever preparing a laminate according to themethod disclosed in EP-A1-0586843, it is necessary to age the saidlaminate before use, in particular when foreseen for the packaging offood.

In international patent application No. PCT/DK 96/00319, publication No.WO-A-97/03821 (which was not available to the public on the prioritydate of the present application), the present applicant suggested tobring the catalyst into contact with the adhesive by incorporating thecatalyst in one of the films to be attached to each other by means ofthe adhesive. Although this reduces the curing time considerably andresults in a low migration of amine compounds so far as laminates withno ink between the films are concerned, the suggestion is notrecommendable when the catalyst-containing film has to be printed withink, because the layer of ink will prevent a diffusion of catalyst fromthe film to the adhesive.

The object of the present invention is to provide a method for thepreparation of a laminate having ink on one or both films to be attachedto each other, whereby the laminate is ready for further treatmentcomparatively soon after its preparation, without any long-lastingageing period.

According to the invention, this is achieved by

a) providing an ink comprising a catalyst suitable for accelerating thecuring of an adhesive for films,

b) applying to a first film one or more completely or partially coveringlayers of the said ink,

c) laminating a second film to the first film inked surface, using anadhesive, the curing of which is accelerated by the catalyst.

The present invention relates to the laminate obtainable by this methodas well. Furthermore, the invention relates to the film being coatedwith one or more entirely or partially covering layers of ink obtainableafter step b) of the method.

Soon after the preparation of the laminate according to he invention,the measurable amine migration is comparatively low, and, in addition, afairly satisfactory mutual adherence of the films is obtained shortlyafter the preparation of the laminate. Due to the quick-curing of theadhesive used as well as the low amine migration, the laminate preparedneeds no or only moderate ageing before further treatment. Especiallywithin the food industry a low amine migration is in great demand, and,when in use, the laminate should show no amine migration at all. Thefact that the laminate prepared may be exposed to further treatmentshortly after its preparation is advantageous from a producer's point ofview, inter alia, because this will reduce the need for storage room,and, sales of the laminate prepared can start sooner after production.

Moreover, a laminate according to the invention has a reduced tendencyto telescoping, i.e. shear of the films relative to each other.

The term “ink” used in the present description and claims refers to bothpigmented and un-pigmented inks and varnishes as well as clear andcoloured varnishes. For the application of the inks methods known fromthe prior art, e.g. gravure, flexography or offset printing, may beused.

The ink employed may be of any suitable type, e.g. nitrocellulose, PVB,acrylic, acrylate or combinations thereof. In addition, the inks may beoffset inks, e.g. oil-based or water-based. The ink may have any colouror be uncoloured.

The inks may be based on any solvent, in particular, they may bedilutable in ethanol, isopropanol, ethylacetate, methylethylketone,acetone, water or combinations thereof. Furthermore, solvent-free inksmay be used.

The inks used according to the invention may be single-component,dual-component and/or even three-component inks. Single-component inksare immediately employable for printing, possibly after having beendiluted by a suitable solvent. Dual-component inks are foreseen to bemixed immediately before use, whereby the first component comprises astandard ink and the second component the catalyst suitable foraccelerating the curing of an adhesive for films.

Any adhesive curing in the presence of a catalyst may be used. Preferredadhesives are of the polyurethane type and alcohol-based or water-basedimine-epoxy systems. Polyurethane adhesives may be single-componentsystems or dual-component systems. A polyurethane adhesive comprises atleast one organic polyisocyanate and at least one polyol, thepolyisocyanate comprising two or more isocyanate groups, and the polyoltwo or more hydroxy groups. The polyol may be a polyester polyol, apolyether polyol or a polyetheresterpolyol. The polyisocyanate may be apre-polymer.

Typical examples of diisocyanates for polyurethane adhesives are1,6-hexanediisocyanate, 1,10-decanediisocyanate,1,3-cylopentanediisocyanate, 1,4-cyclohexanediisocyanate,1-isocyanat-3,3,5-trimethyl-3 or 5-isocyanatomethancyclohexane, 4,4′-,2,4′- and 2,2′-di-isocyanate diphenylmethane,1,5-nanhtalenediisocyanate, 4,4′-diisocyanate-dicyclohexylmethane,1,4-di-isocyanate-benzene and/or 2,4- or 2,6-diisocyanate-tolu-ene.Pre-polymers of polyisocyanates may be achieved by reacting the abovediisocyanates with a polyvalent alcohol.

Typical examples of polyols for polyurethane adhesives are alkanediolswith linear or branched carbon chains, e.g. ethylene glycol,1,2-dihydroxypropane, 1,3-dihydroxypropane, 1,4-dihydroxybutane,1,6-dihydroxyhexane and neopentylglycol. In case of the polyol being apolyetherpolyol, this is usually obtained by adding ethylene oxid to acompound comprising two or more active hydrogen atoms in a molar ratioof 30 to 90%. Suitable examples of compounds comprising two or moreactive hydrogen atoms are ethylene glycol, diethylene glycol, propyleneglycol, dipropylene glycol, glycerin, trimethylol-propane,pentaerythritol, sorbitol, saccharose or the like. Instead ofethylenoxid e.g. propylenoxid and butylenoxid may be used. In general,polyetherpolyol has a molecular mass in the range from 1,000 to 20,000,especially between 1,000 and 5,000.

Polyurethane adhesives used for single-component systems comprise e.g.pre-polymers of polyisocyanates and polyols, the said adhesive curingwhen reacted with water. Examples of this may be found in German patentNo. 2549227 and European patent application No. 0586843 A1.

Dual-component systems either consist of pre-polymers with freeisocyanate groups curing by means of a hydroxy group bearingcross-linking agent, or of pre-polymers with free hydroxy groups, curingby means of isocyanate group bearing cross-linking agents. Examples ofthe first type of dual-component systems are disclosed in Germanlaid-open publication No. 4021113 and in European patent No. 150444.Examples of the latter type of dual-component systems are listed inEuropean patent No. 176726. No matter the type of polyurethane adhesive,these may comprise a solvent, be free of solvents or water-based. Forthe method according to the present invention, it is preferred to use adual-component system, in which the various components are mixedimmediately before their application to the first film.

The catalysts accelerating the curing of the polyurethane adhesives areknown from the prior art. Various substance groups are proved activecatalysts, inter alia Lewis bases and Lewis acids. The most importantLewis bases are tertiary amines, e.g. diazabicyclooctane, triethylamine,dimethylbenzylamine, bisdimethylaminoethylether, tetramethylguanidineand bisdimethylaminomethylphenol. The most important catalyticallyeffective Lewis acids are metallo-organic compounds, especially tenorganic compounds, like tindiethylhexanoate, dibutyltindilaurate,dibutyltinbisdodecylmercaptide and lead-phenylethyldithiocarbaminate.Presumably, the metallo-organic compounds are activating the isocyanategroups thus making these electrophile, whereas the bases first andforemost increase the nucleophile properties of the OH group.

Other known catalysts are alkaline salts of organic acids and phenols,like phenol-Mannich-bases, phosphine and phospholinoxide.

Also ε-caprolactam has catalytic properties, and in the presentinvention the use of ε-caprolactam, which is available as a powder,flakes or in a fluid form, is preferred as a catalyst.

The catalyst itself may be an appropriate mixture of several differentcatalysts so as to obtain an optimal curing in relation to various typesof adhesive

Usually, the catalyst content of ink is up to 20%, however, a catalystcontent of approximately 5% of the solid content of the ink proved to beparticularly suitable.

The catalyst, per se, may be added to the ink immediately or, as apre-solution, optionally with use of a surfactant to obtain a homogenousdistribution in the ink. The catalyst preferred, ε-caprolactam, ispreferentially added to the ink as a pre-solution. Appropriately, thepre-solution comprises equal parts of ε-caprolactam and isopropanoland/or water.

The first film may partially or entirely be made of a plastic material,preferably chosen from polyesters, in particular orientated polyesters;polyolefines, e.g. polyethylenes and orientated polypropylenes or castpolypropylenes; polyamides, in particular cast polyamides or orientatedpolyamides; and cellophane.

The second film may be of the same type as the first film.Preferentially, however, the second film consists entirely or partiallyof aluminium-containing materials, metallized plastics, paper,metallized paper, or plastic materials, in particular polyethylene andpolypropylene.

Both the first and the second film may incorporate a catalyst for anadhesive. Using a film with an incorporated catalyst is particularlyrecommendable in case of only partial application of ink to the film. Inthis way, a catalyzed curing also takes place outside the areas of thefilm covered with catalyst-containing ink.

In cases when a film without an incorporated catalyst is used along withpartial application of a catalyst-containing coloured ink it isrecommendable to apply a completely covering layer of clear varnish orunpigmented catalyst-containing ink to the first film. In this way, acatalyzed curing is ensured outside the areas of the film originallycovered with the coloured ink as well.

Multi-colour printing is well-known in the prior art, i.e. applyingseveral layers of different inks consecutively or on top of each other.All applied inks may contain a catalyst, so that catalyzed curing istaking place within all areas of the laminate. If at last a completelycovering ink is applied, a so-called background colour, only that inkneeds to contain a catalyst to ensure a catalyzed curing within allareas of the laminate.

The first and/or the second film may be a laminate, per se. Inparticular when using a metallo-film in the laminate, it may beadvantageous to coat the metallo-film with a plastic f prior to applyingthe ink.

When the catalyst-containing ink has been applied to the first film thisis laminated to the second film. The adhesive may either be applied tothe first or the second film, prior to their being attached together orbe added simultaneously. Preferably, the adhesive is applied to thefirst ink covered film.

The laminates prepared may be used for the wrapping and packaging offood, i.e. they get into immediate contact with food. Therefore, it mustbe ensured that no noxious substances are transferred to the food, or atleast the extent of the transfer must be non-toxic. ε-caprolactam is acomparatively non-toxic compound, which is released for the market bythe authorities for use in materials getting into contact with food.Since the catalyst is placed between the two films of the laminate, itwill rot set into immediate contact with the food.

Without any intention of limiting the invention to any specifictheoretical explanation, it is assumed, that the catalyst in the inkdiffuses and penetrates the adhesive layer thus accelerating the curingof the adhesive.

In the following, the present invention will be illustrated by means ofexamples, however, these examples are not to be considered as alimitation of the scope of the protection.

EXAMPLE 1

Two inks, one with and one without a catalyst, were applied to separateareas of a first film. After a few days of storage, an adhesive wasapplied to the first film, and a second film was laminated thereto.After the lamination, the adherence and the amine migration was measuredat certain intervals.

The first film was a corona-treated 30 μm polypropylene film (MobilMB400), and the second film was a 50 micron polyethylene film (Borealis0601 standard). The adhesive was a MorFree 403a/C79 mixed in the ratio100:65 and applied in a density of 1.5 g/m².

The ink was a white “Resino Reversal 142” type from Resino A/S having asolid content of 60% (w/w) The catalyst-containing ink labelled “A1” wasprepared by adding a solution of 300 g ε-caprolactam flakes dissolved in300 ml of isopropanol into 10 kg of the above ink. During 14 days ofstorage, the viscosity of the catalyst-containing ink remainedessentially unchanged. The ink without any catalyst was labelled B1.Both inks were applied in a layer of 1.5 μm.

The adherence was measured according to DIN 53357 (unit: N per 15 mm),whereas the amine migration was measured as the migration of primaryaromatic amines (e.g. 2,4-toluenediamine and4,4′-diamino-diphenyl-methane), by a process wherein the amine iscoupled to N-(1-naphthyl)-ethylenediamine-dihydrochloride for theformation of a coloured addition product. The formation of the additionproduct was measured as the absorbance at a wave length of 550 nm. Theconcentration was measured in μg/dm².

The results are indicated in Table Ia and Ib below:

TABLE Ia ADHERENCE 2 hours 4 hours 6 hours 1 day 3 days 8 days A1 ink0.02 0.15 1.10 4.00 4.50 4.60 B1 ink 0.01 0.04 0.09 1.80 4.00 4.00

TABLE Ib AMINE MIGRATION 1 day 3 days 5 days 9 days A1 ink 32 10 1.0 0.3B1 ink 63 26 9.5 4.5

The tables show that the use of a catalyst-containg ink accelerates thecuring of the laminates and makes the amine migration decrease sooner.Furthermore, considerably lower amine migration values are obtained whenusing a catalyst-containing ink compared to the results obtainable whenusing an-ink without a catalyst, at least within the observed timeperiod.

EXAMPLE 2

The same procedure was adopted as in Example 1, only an unpigmented ink(trademark: Resino Reversal) having a solid content of 30% (w/w) wasused. The catalyst-containing ink was labelled A2 and prepared by addinga solution of 62.5 g ε-caprolactam flakes dissolved in 62.5 gisopropanol to 10 kg of the ink. The ink without any catalyst waslabelled B2. The adhesive is as stated in Example 1.

Measurement of the adherence and the amine migration followed asdescribed in Example 1. The results are indicated in Table IIa and IIbbelow:

TABLE IIa ADHERENCE 2 hours 4 hours 6 hours 1 day 3 days 8 days A1 ink0.05 1.00 2.10 3.40 4.10 4.50 B1 ink 0.01 0.06 0.11 2.20 3.30 3.90

TABLE IIb AMINE MIGRATION 1 day 3 days 5 days 9 days A1 ink 24 6 0.9 0.3B1 ink 60 26 10.5 3.6

The tables show that the use of a catalyst-containing ink acceleratesthe curing of the laminates and makes the amine migration decreasesooner. Furthermore, considerably lower amine migration values areobtained when using a catalyst-containing ink compared with the resultsobtained when using an ink without a catalyst, at least within theobserved time period.

EXAMPLE 3

In this example, the adherence effect from various concentrations ofε-caprolactam in the ink was studied at room temperature and at anelevated temperature, at various time intervals after the lamination.

The first film was an oriented 30 μm polypropylene film from UCB,whereas the second was a 50 μm polyethylene film (trademark SengewaldK-203). The adhesive was a Novacote SF782 A and CA 375 mixed in te ratio100:36. The adhesive was applied in a layer of 1.7-1.9 g/m² to the firstink-treated film.

The ink was white (Resino Reversal 142) and supplemented with variousconcentrations of ε-caprolactam.

Two samples from each run were stored at room temperature and for 6hours at 35° C., followed by room temperature, respectively, whereuponthe adherence was measured according to DIN 53357. The results arelisted in Tables IIIa and IIIb.

TABLE IIIa Storing at Room Temperature Adherence Concentration of afterafter after after after ε-caprolactam 1 2 4 6 24 % (w/w) hour hourshours hours hours 0.0 <0.3 0.5 1.6 2.3 4.7 T 1.25 0.6 1.3 2.5 4.7 T 3.5T 2.5 1.7 2.7 3.1 T 3.5 T 4.7 T 5.0 3.5 4.7 T 4.8 T 4.8 T 3.5 T P + T T= Tear P = Peel

TABLE IIIb Storing for 6 hours at 35° C., Followed by Storing at RoomTemperature Adherence Concentration of after after after after afterε-caprolactam 1 2 4 6 24 % (w/w) hour hours hours hours hours 0.0 0.62.0 4.3 T 5.0 T 4.5 T 1.25 2.0 4.6 T 4.2 T 3.2 T 4.6 T 2.5 3.8 T 4.7 T4.9 T 4.9 T 4.7 T 5.0 3.7 T 4.1 T 4.8 T 4.4 T 4.8 T T = Tear

From the tables IIIa and IIIb it may be deduced that an increasedconcentration of ε-caprolactam accelerates the curing of the laminatesproduced. Furthermore, also storing at increased temperatures results incuring acceleration. One hour after production, the adherence of thelaminates containing 2.5 and 5.0% (w/w) ε-caprolactam, respectively, isadequate for further treatment of the laminates. By way of comparison,laminates with no ε-caprolactam must be stored at least 4 hours beforeuse.

EXAMPLE 4

A first polyester-type film and a second film, which is a laminatebetween an aluminium film and a polyethylene film, was provided.

The first film was coated with a transparent ink (“Resino Reversal”without pigment) in a 1.5 μm layer, whereupon an adhesive (“Novacote SF782A” and “CA 375”, mixed in the ratio of 100:40) was applied in anamount of 1.7-1.9 g/m². With the aluminium side against the adhesive,the second film was laminated thereto.

Using increasing amounts of ε-caprolactam, the adherence was measuredafter storing at room temperature (Table IVa) as well as after storing 6hours at 35° C. followed by room temperature (Table IVb). The resultsare shown in the tables below.

TABLE IVa Storing at Room Temperature Adherence (N/15 mm) Concentrationof after after after after after ε-caprolactam 1 2 4 6 24 % w/w hourhours hours hours hours 0.0 0.2 0.5 2.0 1.9 3.4   P + T 1.25 0.5 1.0 2.32.9 4.9   P + T 2.5 0.6 2.1 2.8 3.7 3.1   P + T P + T 5.0 2.0 3.3 4.03.0 3.6 T P + T P + T T = Tear P = Peel

TABLE IVb Storing for 6 hours at 35° C., Followed by Storing at RoomTemperature Adherence (N/15 mm) Concentration of after after after afterafter ε-caprolactam 1 2 4 6 24 % (w/w) hour hours hours hours hours 0.00.5 2.1 2.8 3.2   3.2   P + T P + T 1.25 0.7 2.5 5.3 3.4 T 4.9 T P + T2.5 1.8 2.7 3.3 3.0   3.0   P + T P + T P + T 5.0 2.8 4.0 3.4 3.5   3.6T P + T P + T P + T P = Peel T = Tear

From the tables IVa and IVb it may be deduced that an increasedconcentration of ε-caprolactam accelerates the curing of the laminatesproduced. Furthermore, also storing at increased temperatures results incuring acceleration.

The laminate produced has a metallic appearance. It may be used forpackaging aromatic products, e.g. coffee.

What is claimed is:
 1. A method for preparing a laminated food-wrappingfilm which minimizes or eliminates amine migration, which methodcomprises: a) providing an ink comprising a catalyst suitable foraccelerating curing of a polyurethane-type adhesive for films, whereinthe ink is a member selected from the group consisting ofnitrocellulose, PVB, acrylic, acrylate or combinations thereof, b)applying to a first film one or more completely or partially coveringlayers of the ink, and c) laminating a second film to the first film'sinked surface using a polyurethane-type adhesive, the curing of which isaccelerated by the catalyst, to form the laminated food-wrapping film.2. A method according to claim 1, wherein the catalyst is a memberselected from the group consisting of ε-caprolactam anddibutyltindilaurate, and the catalyst is present in an amount notexceeding 20% by weight based upon the solid content of the ink.
 3. Amethod according to claim 1, wherein the catalyst is present in anamount of approximately 5% by weight, based upon the solid content ofthe ink.
 4. A method according to claim 1, wherein the adhesive isapplied to the first film coated with ink, the first film beingsubsequently laminated to the second film.
 5. A method according toclaim 1 wherein the first film is partly covered with one or more layersof the ink.
 6. A method according to claim 1, wherein the first film isentirely or partially made of a plastic material.
 7. A method accordingto claim 6, wherein the plastic material is a member selected from thegroup consisting of a polyester, a polyolefine, a polyamide andcellophane.
 8. A method according to claim 6, wherein the polyester isan oriented polyester, the polyolefine is a polyethylene, orientedpolypropylene or cast polypropylene and the polyamide is cast polyamideor an oriented polyamide.
 9. A method according to claim 1, wherein thesecond film is entirely or partially made of aluminum containingmaterial, metallized plastic, paper, metallized paper, or a plasticmaterial.
 10. A method according to claim 9, wherein the plasticmaterial is polyethylene or polypropylene.
 11. A method according toclaim 1, wherein the first and/or the second film is a laminate per se.12. A laminate obtainable by means of the method according to claim 1.13. A quick-cured laminated composite film which is useful as afood-wrapping film and which comprises a first film and a second film,inside surfaces of which are secured together by a polyurethane-typeadhesive cured in situ and in contact with ink completely or partiallycovering the inside surface of the first film and comprising a catalystfor accelerating cure of the adhesive, wherein the ink is a memberselected from the group consisting of nitrocellulose, PVB, acrylic andacrylate, and said composite film is one which minimizes or eliminatesamine migration therethrough.
 14. A quick-cured laminated compositeaccording to claim 13 wherein the first film is entirely or partiallymade of a plastic material.
 15. A quick-cured laminated composite filmaccording to claim 14, wherein the plastic material is a member selectedfrom the group consisting of a polyester, a polyolefine, a polyamide andcellophane.
 16. A quick-cured laminated composite film according toclaim 14 wherein the polyester is an oriented polyester, the polyolefineis a polyethylene, oriented polypropylene or cast polypropylene and thepolyamide is cast polyamide or an oriented polyamide.
 17. A quick-curedlaminated composite film according to claim 13 wherein the second filmis entirely or partially made of aluminum containing material,metallized plastic, paper, metallized paper, or a plastic material. 18.A quick-cured laminated composite film according to claim 17, whereinthe plastic material is polyethylene or polypropylene.